In the construction of gas turbines efforts are being made to ever improve the characteristics of thermally highly loaded structural components, especially the turbine blades of the first turbine stage. Thus, it is known to obtain improvements in the resistance against high temperature loads by the application of highly effective protection layers against corrosion. It is known that metal coatings or protective layers made of an MCrAlY alloy are especially suitable for this purpose. In the just mentioned alloy the M stands for nickel, cobalt, or an alloy of the two. Under special circumstances iron may also take the place of the letter M.
The protection effect with regard to the surface to be protected, is based on the fact that the chromium and aluminum form oxides at these high temperatures, namely Cr.sub.2 O.sub.3 and Al.sub.2 O.sub.3. These oxides form protective films which prevent any further oxidation.
The alloys used conventionally comprise about 15% to 25% of chromium, 10% to 15% of aluminum, 0.2% to 0.5% of yttrium, and the rest being represented by the M, as mentioned above, whereby the indicated percentages are weight percentages. The proportion of aluminum and chromium should be as high as possible in order to make sure that the above mentioned protecting effect by way of forming an oxide layer can function to the required extent. Conventional application methods employ the thermal spraying as well as physical vapor deposition techniques, whereby the required proportion of chromium, aluminum, and yttrium in the layer is obtained. A disadvantage of thermal spraying and physical vapor deposition methods is their high production costs. Thus, tests have been made to apply these protective coatings by a dispersion coating technique because dispersion coating is substantially more economical compared to the above mentioned two methods. However, dispersion coating has also disadvantages. Thus, conventional dispersion coating methods could achieve only small insertion rates of the suspension powder in the metal matrix. The insertion rates are in the order of about 20% by volume, whereby it is not possible to achieve the required high chromium and aluminum content proportion. As a result, the protective coating does not have the required quality. Useful protection coating qualities would require a proportion of more than 40% by volume of the chromium and aluminum in order to achieve the same coating or film quality as can be achieved by means of physical vapor deposition or plasma spraying methods.
An article published in the trade journal "Plating and Surface Finishing" of October 1986, page 42, describes a method which is intended to avoid the above mentioned disadvantages of the dispersion coating method. In this known method a suspension filled drum having partially porous walls is rotated in an electrolytic bath. The substrates to be coated are attached inside the rotating drum. Relatively high insertion rates are achieved by this method. However, the rotating drum method also has a disadvantage, namely that the resulting coating or film is very non-uniform. A particularly undesirable characteristic of the rotating drum method is seen in the fact that substantial wart-like depositions are formed. In connection with the coating of turbine blades, the known method results in thicker coatings along the blade edges than in the central blade areas. This disadvantageous effect could, theoretically, possibly be avoided by mounting screens or so-called shutters inside the drums. However, such possibility is really not practical because it is likely to cause electrical short circuits through the electrolyte. Thus, the problem cannot be easily avoided. Another disadvantage of the rotating drum dispersion method resides in the fact that it is rather time consuming and therefore is not suitable for an economical large scale production.
Another substantial disadvantage of conventional dispersion coating methods resides in the fact that frequently a very porous layer structure is obtained which additionally has a rough surface which is dotted with dendritic patterns so that the desired corrosion protection is rather non-uniform over the surface area to be protected.